Pathways of Methane Transformation over Copper-Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy

The reaction of methane with copper-exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic Cu^II species. Several types of surface-bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono- and bidentate formates, Cu^I monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper-exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.     

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection

Copper(II)-containing mordenite (CuMOR) is capable of activation of C−H bonds in C₁-C₃ alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with Cu^I sites, formed upon reduction of Cu^II-oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.     

The Effect of the Active‐Site Structure on the Activity of Copper Mordenite in the Aerobic and Anaerobic Conversion of Methane into Methanol

Samples of the zeolite mordenite with different Si/Al ratios were used to synthesize materials with monomeric and oligomeric copper sites that are active in the direct conversion of methane into methanol. A comparison of two reactivation protocols with oxygen (aerobic oxidation) and water (anaerobic oxidation), respectively, revealed that such copper–oxo species possess different reactivity towards methane and water. We show for the first time that oligomeric copper species exhibit high activity under both aerobic and anaerobic activation conditions, whereas monomeric copper sites produce methanol only in aerobic processes.     

Copper‐Catalyzed 1,2‐Methoxy Methoxycarbonylation of Alkenes with Methyl Formate

Reported here is a copper‐catalyzed 1,2‐methoxy methoxycarbonylation of alkenes by an unprecedented use of methyl formate as a source of both the methoxy and the methoxycarbonyl groups. This reaction transforms styrene and its derivatives into value‐added β‐methoxy alkanoates and cinnamates, as well as medicinally important five‐membered heterocycles, such as functionalized tetrahydrofurans, γ‐lactones, and pyrrolidines. A ternary β‐diketiminato‐Cu^I‐styrene complex, fully characterized by NMR spectroscopy and X‐ray crystallographic analysis, is capable of catalyzing the same transformation. These findings suggest that pre‐coordination of electron‐rich alkenes to copper might play an important role in accelerating the addition of nucleophilic radicals to electron‐rich alkenes, and could have general implications in the design of novel radical‐based transformations.     

Mechanism of methanol conversion on ZrO<Subscript>2</Subscript> and 5% Cu/ZrO<Subscript>2</Subscript> according to in situ IR spectroscopic data

It is demonstrated by in situ IR spectroscopy that, in methanol conversion on ZrO₂ and 5% Cu/ZrO₂ catalysts, methoxy groups are present on the catalyst surface, which result from O-H or C-O bond breaking in the methanol molecule. Two types of formate complexes, localized on ZrO₂ and CuO, are also observed. The formate complexes form via the oxidative conversion of the methoxy groups. There are two types of linear methoxy groups. First-type linear methoxy groups condense with the formate complex located on CuO to yield methyl formate and then CO and H₂. Second-type methoxy groups appear as intermediate products in the formation of dimethyl ether. The main hydrogen formation reactions are the recombination of hydrogen atoms (which result from the interconversion of surface complexes) on copper clusters and the decomposition of methyl formate. The source of CO₂ in the gas phase is the formate complex, and the source of CO is methyl formate. The effect of water vapor and oxygen the surface reactions and product formation is discussed.     

Extra‐Framework Aluminum‐Assisted Initial C−C Bond Formation in Methanol‐to‐Olefins Conversion on Zeolite H‐ZSM‐5

Surface methoxy species bound to an extra‐framework Al (SMS‐EFAL) was unambiguously identified by advanced ¹³C‐{²⁷Al} double‐resonance solid‐state NMR technique in the methanol‐to‐olefins reaction on H‐ZSM‐5 zeolite. The high reactivity of the SMS‐EFAL leads to the formation of surface ethoxy species and ethanol as the key intermediates for ethene generation in the early reaction stage. A direct route for the initial C?C bond formation in ethene was proposed and corroborated by density functional theory calculations.     

Properties of surface compounds in methanol conversion on γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Data of in situ IR spectroscopy

In situ IR spectroscopic studies show that a formate, an aldehyde-like complex, and bridging and linear methoxy groups exist on the alumina surface involved in methanol conversion. In the absence of methanol in the gas phase, the interaction between two bridging methoxy groups yields dimethyl ether in the gas phase. When methanol is present in the gas phase, it interacts with methoxy groups on the surface. This reaction makes the main contribution to the formation of dimethyl ether. The linear methoxy group undergoes conversion via several routes. The main route is desorption with methanol formation in the gas phase, and no more than 10% of the linear methoxy groups are converted into formate and aldehyde, which are CO₂ sources in the gas phase. In the absence of methanol in the gas phase, the conversion rate of the methoxy groups is independent of the presence of water and oxygen. A scheme of the surface reactions is suggested to explain the conversion of the methoxy groups.     

The application of diffuse reflectance infrared spectroscopy and temperature-programmed desorption to investigate the interaction of methanol on eta-alumina

The adsorption of methanol and its subsequent transformation to form dimethyl ether (DME) on a commercial grade eta-alumina catalyst has been investigated using a combination of mass selective temperature-programmed desorption (TPD) and diffuse reflectance infrared spectroscopy (DRIFTS). The infrared spectrum of a saturated overlayer of methanol on eta-alumina shows the surface to be comprised of associatively adsorbed methanol and chemisorbed methoxy species. TPD shows methanol and DME to desorb with respective maxima at 380 and 480 K, with desorption detectable for both molecules up to ca. 700 K. At 673 K, infrared spectroscopy reveals the formation of a formate species; the spectral line width of the antisymmetric C-O stretch indicates the adoption of a high symmetry adsorbed state. Conventional TPD using a tubular reactor, combined with mass spectrometric analysis of the gas stream exiting the IR cell, indicate hydrogen and methane evolution to be associated with formation of the surface formate group and CO evolution with its decomposition. A reaction scheme is proposed for the generation and decomposition of this important reaction intermediate. The overall processes involved in (i) the adsorption/desorption of methanol, (ii) the transformation of methanol to DME, and (iii) the formation and decomposition of formate species are discussed within the context of a recently developed four-site model for the Lewis acidity of eta-alumina.     

Molecular CuII‐O‐CuII Complexes: Still Waters Run Deep

Research on O₂ activation at ligated Cu^I is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a Cu^II‐O‐Cu^II species, which is more special than it appears at first sight: a single oxo ligand between two Cu^II centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when Cu^II‐O‐Cu^II cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu‐grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features.     

A Highly Stable Copper‐Based Catalyst for Clarifying the Catalytic Roles of Cu0 and Cu+ Species in Methanol Dehydrogenation

Identification of the active copper species, and further illustration of the catalytic mechanism of Cu‐based catalysts is still a challenge because of the mobility and evolution of Cu⁰ and Cu⁺ species in the reaction process. Thus, an unprecedentedly stable Cu‐based catalyst was prepared by uniformly embedding Cu nanoparticles in a mesoporous silica shell allowing clarification of the catalytic roles of Cu⁰ and Cu⁺ in the dehydrogenation of methanol to methyl formate by combining isotope‐labeling experiment, in situ spectroscopy, and DFT calculations. It is shown that Cu⁰ sites promote the cleavage of the O?H bond in methanol and of the C?H bond in the reaction intermediates CH₃O and H₂COOCH₃ which is formed from CH₃O and HCHO, whereas Cu⁺ sites cause rapid decomposition of formaldehyde generated on the Cu⁰ sites into CO and H₂.     

Structure of copper sites in zeolites examined by Fourier and wavelet transform analysis of EXAFS

Copper-exchanged zeolites are a class of redox-active materials that find application in the selective catalytic reduction of exhaust gases of diesel vehicles and, more recently, the selective oxidation of methane to methanol. However, the structure of the active copper-oxo species present in zeolites under oxidative environments is still a subject of debate. Herein, we make a comprehensive study of copper species in copper-exchanged zeolites with MOR, MFI, BEA, and FAU frameworks and for different Si/Al ratios and copper loadings using X-ray absorption spectroscopy. Only obtaining high quality EXAFS data, collected at large k-values and measured under cryogenic conditions, in combination with wavelet transform analysis enables the discrimination between the copper-oxo species having different structures. The zeolite topology strongly affects the copper speciation, ranging from monomeric copper species to copper-oxo clusters, hosted in zeolites of different topologies. In contrast, the variation of the Si/Al ratio or copper loading in mordenite does not lead to significant differences in XAS spectra, suggesting that a change, if any, in the structure of copper species in these materials is not distinguishable by EXAFS.

The structure of copper-oxo species hosted in zeolites of various topology has been examined using wavelet and Fourier transform analysis of Cu K-edge EXAFS spectra.     

Reactivity of C1 surface species formed in methane activation on Zn-modified H-ZSM-5 zeolite

Solid-state (13)C magic angle spinning (MAS) NMR spectroscopy investigations identified zinc methyl species, formate species, and methoxy species as C(1) surface species formed in methane activation on the zeolite Zn/H-ZSM-5 catalyst at T≤573 K. These C(1) surface species, which are possible intermediates in further transformations of methane, were prepared separately by adsorption of (13)C-enriched methane, carbon monoxide, and methanol onto zinc-containing catalysts, respectively. Successful isolation of each surface species allowed convenient investigations into their chemical nature on the working catalyst by solid-state (13)C MAS NMR spectroscopy. The reactivity of zinc methyl species with diverse probe molecules (i.e., water, methanol, hydrochloride, oxygen, or carbon dioxide) is correlated with that of organozinc compounds in organometallic chemistry. Moreover, surface formate and surface methoxy species possess distinct reactivity towards water, hydrochloride, ammonia, or hydrogen as probe molecules. To explain these and other observations, we propose that the C(1) surface species interconvert on zeolite Zn/H-ZSM-5. As implied by the reactivity information, potential applications of methane co-conversion on zinc-containing zeolites might, therefore, be possible by further transformation of these C(1) surface species with rationally designed co-reactants (i.e., probe molecules) under optimized reaction conditions.     

Magic angle spinning29Si and27Al NMR study of mordenite dealumination

Magic angle spinning²⁹Si NMR presents a rapid qualitative method of assessing the degree of dealumination of sodium mordenite modified by acid leaching and heat treatment. A quantitative determination of the²⁹Si coordination sphere is hampered by the overlap of chemical shift ranges for Si?O?Al and Si?O?H species. MAS²⁷Al NMR indicates the presence of octahedrally coordinated Al in interstitial sites in all treated samples. On treating mordenite samples at high temperatures (～700°C) much of the Al becomes invisible to the NMR experiment because of its location in sites of low symmetry and large quadrupole coupling constant.     

Importance of Methane Chemical Potential for Its Conversion to Methanol on Cu-Exchanged Mordenite

Copper-oxo clusters exchanged in zeolite mordenite are active in the stoichiometric conversion of methane to methanol at low temperatures. Here, we show an unprecedented methanol yield per Cu of 0.6, with a 90–95 % selectivity, on a MOR solely containing [Cu₃(μ-O)₃]²⁺ active sites. DFT calculations, spectroscopic characterization and kinetic analysis show that increasing the chemical potential of methane enables the utilization of two μ-oxo bridge oxygen out of the three available in the tricopper-oxo cluster structure. Methanol and methoxy groups are stabilized in parallel, leading to methanol desorption in the presence of water.     

Picolinamides as Effective Ligands for Copper‐Catalysed Aryl Ether Formation: Structure–Activity Relationships,Substrate Scope and Mechanistic Investigations

The use of picolinic acid amide derivatives as an effective family of bidentate ligands for copper‐catalysed aryl ether synthesis is reported. A fluorine‐substituted ligand gave good results in the synthesis of a wide range of aryl ethers. Even bulky phenols, known to be very challenging substrates, were shown to react with aryl iodides with excellent yields using these ligands. At the end of the reaction, the first examples of end‐of‐life Cu species were isolated and identified as Cu^II complexes with several of the anionic ligands tested. A preliminary mechanistic investigation is reported that suggests that the substituents on the ligands might have a crucial role in determining the redox properties of the metal centre and, consequently, its efficacy in the coupling process. An understanding of these effects is important for the development of new efficient and tunable ligands for copper‐based chemistry.     

Spectroscopic study of the properties of surface compounds in methanol conversions on Cu/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The reactions of methanol on the (10% Cu)/γ-Al₂O₃ surface were studied by the spectrokinetic method (simultaneous measurements of the conversion rates of surface compounds and the product formation rates). Bridging and linear methoxy groups result from the interaction of methanol with surface hydroxyl groups. Formate and aldehyde-like complexes form by the oxidative conversion of the linear methoxy groups. Hydrogen forms via the recombination of hydrogen atoms on copper clusters, and the hydrogen atoms result from interconversions of surface compounds. The source of CO₂ in the gas phase is the formate complex, and the source of CO is the aldehyde complex. In the absence of methanol in the gas phase, dimethyl ether forms by the interaction between two bridging methoxy groups. When present in the gas phase, methanol reacts with methoxy groups on the surface. The roles of oxygen and water vapor in the conversions of surface compounds are discussed.     

Experimental and Theoretical Studies of Bromination of Diethyl 2,4,6‐Trimethyl‐1,4‐dihydropyridine‐3,5‐dicarboxylate

A reaction of diethyl 2,4,6‐trimethyl‐1,4‐dihydropyridine‐3,5‐dicarboxylate with 1, 2, and more equivalents of N‐bromosuccinimide (NBS) in methanol was investigated by NMR spectroscopy at a temperature interval ranging from 25 to 40°C. The reaction was found to proceed through several steps. The structures of the intermediates diethyl 3‐bromo‐2,4,6‐trimethyl‐3,4‐dihydropyridine‐3,5‐dicarboxylate, diethyl 3‐bromo‐2‐methoxy‐2,4,6‐trimethyl‐1,2,3,4‐tetrahydropyridine‐3,5‐dicarboxylate, and diethyl 3,5‐dibromo‐2‐methoxy‐2,4,6‐trimethyl‐2,3,4,5‐tetrahydropyridine‐3,5‐dicarboxylate were identified by multinuclear (¹H, ¹³C, and ¹⁵N) NMR spectral data. The optimal structures of all species participating in the reaction as well as changes in their relative energies along with the proposed pathway of the reaction were analyzed by quantum‐chemical calculations. The mechanism of bromination of diethyl 2,4,6‐trimethyl‐1,4‐dihydropyridine‐3,5‐dicarboxylate with NBS in methanol was found to favor the bromination in the 2,6‐methyl side chains as the only products in full agreement with experimental observations.     

Direct Detection of Supramolecular Reaction Centers in the Methanol‐to‐Olefins Conversion over Zeolite H‐ZSM‐5 by 13C–27Al Solid‐State NMR Spectroscopy

Hydrocarbon‐pool chemistry is important in methanol to olefins (MTO) conversion on acidic zeolite catalysts. The hydrocarbon‐pool (HP) species, such as methylbenzenes and cyclic carbocations, confined in zeolite channels during the reaction are essential in determining the reaction pathway. Herein, we experimentally demonstrate the formation of supramolecular reaction centers composed of organic hydrocarbon species and the inorganic zeolite framework in H‐ZSM‐5 zeolite by advanced ¹³C–²⁷Al double‐resonance solid‐state NMR spectroscopy. Methylbenzenes and cyclic carbocations located near Brønsted acid/base sites form the supramolecular reaction centers in the zeolite channel. The internuclear spatial interaction/proximity between the ¹³C nuclei (associated with HP species) and the ²⁷Al nuclei (associated with Brønsted acid/base sites) determines the reactivity of the HP species. The closer the HP species are to the zeolite framework Al, the higher their reactivity in the MTO reaction.     

Mechanism of Aniline Methylation on Zeolite Catalysts Investigated by In Situ 13C NMR Spectroscopy

The alkylation reaction of aniline with methanol on zeolites HY and CsOH/CsNaY was studied by in situ ¹³C NMR spectroscopy under flow and batch conditions. Attention was focused on the identification of intermediates and on the determination of the formation mechanisms of N-methylaniline, N,N-dimethylaniline, and toluidines. To refine the main steps of the reaction, the transformations of the following individual compounds and intermediates, which were detected in the course of alkylation, were studied: dimethyl ether, surface methoxy groups, methylanilinium ions, formaldehyde, and N-methyleneaniline. It was found that N-methylaniline and N,N-dimethylaniline were formed as a result of aniline methylation by methanol dehydration products (methoxy groups or dimethyl ether) on acidic zeolites or as a result of alkylation by formaldehyde or methoxy groups on basic zeolites. Toluidines were formed by the isomerization ofN-methylanilinium ions, which were produced only on acidic zeolites, rather than by the direct alkylation of aniline.     

CO2‐to‐Methanol Hydrogenation on Zirconia‐Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal–Support Interface

Methanol synthesis by CO₂ hydrogenation is a key process in a methanol‐based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO₂ to methanol still remain unknown. A mechanistic study of the hydrogenation of CO₂ on Cu/ZrO₂ is presented. Using kinetics, in situ IR and NMR spectroscopies, and isotopic labeling strategies, surface intermediates evolved during CO₂ hydrogenation were observed at different pressures. Combined with DFT calculations, it is shown that a formate species is the reaction intermediate and that the zirconia/copper interface is crucial for the conversion of this intermediate to methanol.